The invention relates to a gas suspension reactor, hereinafter referred to as of the kind described, comprising a substantially cylindrical vessel mounted with its axis substantially vertical and with a downwardly and inwardly sloping bottom wall, means for introducing granular material to be treated and fuel into the vessel adjacent to the bottom wall, and means for introducing a jet of gas into the vessel centrally up through the bottom wall for reaction with the material and/or for burning out the fuel, and for suspension of the material.
As the time of introduction into the reactor vessel the gas velocity is high, typically 20-35 m/sec, while the average gas velocity through the vessel cross-section typically is lower by a factor of 10. However, the gas velocity is not constant across the vessel cross-section as near the center it is much higher than the average gas velocity and along the vessel wall it is lower than this average. Near the wall the gas velocity is lower than the velocity of fall of the material particles. Consequently, material particles flow back along the wall to the vessel bottom where the particles are reentrained by the central gas jet. By this internal material circulation the material retention time in the reactor is prolonged to 4-5 times the gas through-flow time through the reactor.
In a reactor of reasonable height, retention times of between 5 sec. and 1/2 min. may be obtained, this being sufficient for e.g. calcining calcium carbonate in cement raw materials.
In other processes such as calcining of phosphate, or reduction of iron ore, considerably longer retention times in the range 3-5 min. are required.
Consequently it is the object of the invention to provide a gas suspension reactor with prolonged material retention time.
The object is achieved by a reactor of the kind described which, according to the invention, is characterized in that, interspersed at intervals up along the inside of the vessel there are mounted means for making the material entraining gas flow through the vessel rotate about the vessel axis so that the gas flow follows a helical path, and means for causing material falling down along the vessel walls to be displaced inwardly towards the vessel axis.
By the rotating movement, the material is flung towards the cylindrical vessel wall where it is stopped and falls down along the wall towards the vessel bottom. By thus increasing the material circulation inside the reactor the average material retention time is prolonged, but this step alone would cause an undesirably large material concentration at the vessel bottom simultaneously with extensive back-mixing, i.e. mixing of almost finish treated material with fresh untreated material.
This is counteracted by the means displacing the material falling along the vessel wall inwards towards the vessel axis where it is resuspended in the rapid gas flow prevailing in that area. Such means are mounted at intervals up through the vessel, and material which has passed such means in the upward direction, i.e. entrained by the gas flow, will only be able to pass to a small extent the same means in the downward direction, i.e. falling along the vessel wall. In this way the unwanted high material concentration at the vessel bottom is replaced by a number of minor local material concentrations dispersed over the length of the vessel.
According to a preferred arrangement, the means creating the rotation of the material entraining gas flow about the vessel axis are guide vane fittings projecting inwardly from the vessel wall. This rotation may, if desired, be supplemented in other ways, e.g. through blowing in an auxiliary gas flow in a direction with a component tangential to the vessel wall.
Further, according to a preferred construction, the means for displacing the material falling down along the wall of the vessel towards the axis of the vessel are rings or other annular fittings, each having a frusto-conical surface sloping inwards and downwards with the conical apex substantially in the vessel axis. The outer diameter of the frusto-conical surface may correspond to the inner diameter of the vessel and the angle between the axis of the vessel and the frusto-conical surface may be no greater than 30.degree..